1. Field of the Invention
The invention relates to a sample holder for an NMR spectrometer for use with liquid samples,
2. Description of the Related Art
One known type of sample holder for microsamples in the microliter range is produced by the applicant as an accessory to the NMR spectrometers.
This known sample holder uses as a sample holder a glass tube with an outer diameter of about 5 mm which has been drawn to a diameter of only about 2.5 mm at its closed end along a distance of about 40 mm. The entire tube is about 160 mm long. Inside the non-drawn section, the inner diameter is about 4 mm. Usually, a liquid microsample is introduced into this tube with a pipette such that at least a substantial part of the narrowed (lower) section of the tube is filled. Because of the length of the tube, the pipette does not reach its closed (lower) end, so that the sample liquid runs down the inner wall. Because of this, a fraction is adsorbed at the surface, and a fraction evaporates. After filling, the tube is closed by a plug at its open end which is not perfectly gas-tight. After some time, an equilibrium becomes established in the inner volume of the sample tube, where saturated sample vapour lies above the liquid volume in the lower (narrowed) section. Sample vapour may, however, possibly escape slowly past the plug, whereby the sample continues to evaporate. At least during the initial phase, this evaporation is favoured by the large covered surface of the inner wall.
Afterwards, the filled tube is inserted into the central bore of a so-called rotor, whose inner diameter is slightly larger than the outer diameter of the tube, whereby the tube is centred. By means of O-rings slid onto it, the tube is held and its axial position fixed. Generally, the rotor essentially consists of two coaxial cylinders with a common central bore but different outer diameters, which are linked by a conical section. Together with the sample tube, this rotor is lowered vertically from above against a gas stream into the room temperature bore of the cryostat of the superconducting magnet of the NMR spectrometer. The conical section of the rotor either rests on top of a corresponding part of a stator provided inside the probe head of the spectrometer on a gas cushion immediately above said stator or is caused to rotate around its axis (now corresponding to the magnet axis), by means of a gas stream. In this rotor position, the filled part of the sample tube is located just at the central region of the magnet coil and is almost directly surrounded by a high frequency coil to excite and detect NMR signals from the sample.
As far as possible, the sample itself should have the shape of an elongate cylinder, with its ends extending beyond the sensitivity range of the RF coil at both ends, to avoid deterioration of the magnetic field homogeneity because of the susceptibility of the sample. On the other hand, it should fill the sensitivity volume of the RF coil as completely as possible, in order to improve the signal-to-noise ratio. Therefore, up to now, sample holders have been used which are derived from a standard sample holder with a non-tapered tube. These may be used during automatic or semi-automatic spectrometer operation, i.e., with an automatic sample changer, robot manipulator and lowering and ejection of the sample using a computer controlled gas jet. The materials used inside the probe head have to fulfil certain criteria, which are essential for NMR spectrometer operation. Their magnetic susceptibility should be as small as possible, in order not to disturb the field homogeneity inside the measuring volume. Their electrical conductivity should be small, and they should cause few dielectric RF losses. Dependent on the type of nucleus under investigation, they must be free of compounds which could cause interfering NMR signals, i.e., generally free of protons and .sup.13 C. Materials often used are glass, quartz and PTFE.
In addition, glass sample tubes have been used with a uniformly small cross section, the opening of the tube being fused after introducing the sample. These tubes are manually installed into the probe head of an NMR spectrometer and again removed manually after the measurement. Using these prior art tubes, sample changing is difficult, and automatic sample changing is impossible.
The applicant's prior art sample holder referred to above, shows deficiencies some of which have been addressed by the uniform cross-section tube. The tube with cross section reduction is costly to produce. Centring is basically worse than with a simple tube. In addition, there is a risk that in the narrowed section the wall thickness is not constant. All these errors directly, and very often disastrously, influence the field homogeneity and therefore the resolution of the spectrometer. The quite large inner volume in combination with a potential leak at the sealing plug reduces the sample amount by evaporation. However, often only one small sample is available for analysis.
FIG. 3 shows a prior art NMR sample holder for microsamples. Rotor 2' has the same dimensions as rotor 2 in FIGS. 1 and 2. The central bore 2'e has a constant cross section which is slightly larger than the outer diameter of the upper section of sample tube 4'. This sample tube 4' has approximately the same total length as the sample tube 4 plus handling member 3 in FIG. 1, i.e., about 180 mm. Its upper section 4' has a diameter of about 5 mm. In the lower section 4'b this is reduced to about 2.5 mm for a length of about 45 mm. The sample liquid 5 occupies a part of this section 4'b. On top there is a rather large vapour-filled inner volume which can be sealed at the open upper end with a plastic or rubber plug 9'. One or more sliding O-rings 8' effect a bearing and axial alignment of sample tube 4' by each compressing a slotted axial extension of the rotor and thereby locally reducing bore 2 e to the outer diameter of tube 4'a.